Throughflow dryers are conventionally known, with a multi-level construction for drying plate-shaped or panel-shaped products, with means, especially rollers or belt-like means, for the transport of the products within the throughflow dryer. The dryer has several dryer zones which comprise a housing and which are arranged one behind another in a transport direction of the products. In the dryer, drying air circulates in an air circulation method, whereby each dryer zone comprises at least one central heat source and several axial blowers arranged vertically over one another in a blower stand. The axial blowers convey the drying air in a direct path into inflow openings of nozzle boxes arranged over and under the transport means for blowing the drying air onto the plate-shaped products, and then sucking in the moisture laden drying air and again circulating it over the at least one heat source.
Throughflow dryers with the above mentioned features have long been known, also see “Die richtige Lösung für die Bauplatten-Industrie”, (“The Right Solution for the Construction Panel Industry”), prospectus of Lindauer DORNIER GmbH, page 6/7, imprint 12/01/LD/02/99.
In the known multi-level throughflow dryers, in which moisture is withdrawn or extracted from the products to be dried by high-temperature tempered or heated air, the moisture laden air proceeds in an air circulation loop as low temperature tempered or heated air to the at least one heat source and thereafter as high temperature tempered air back to the blowers.
Measurements for determining the residual moisture in the products, which leave the throughflow drier at the end of the drying process, have shown that products that pass through the dryer in the lower levels of the throughflow dryer have a smaller residual moisture than products that pass through the throughflow dryer in the upper levels. The cause of the differing residual moisture in the finished dried products is to be searched for in the fact that a portion of the moisture laden tempered circulation air, due to its thermodynamic characteristics, stagnates in the upper levels of the throughflow dryer. Thus, the stagnant circulation air can contain, for example, 40 grams of H20 per 1 kg of air. Because this circulation air comprising a relatively high saturation degree of water is positively again directed to the product by means of the blowers allocated to the upper levels through the blowing boxes of the upper levels, the moist drying air blown onto the product to be dried cannot withdraw or extract the desired water quantity from the products. Accordingly, the dried products exit with a differing proportion or content of residual moisture corresponding to the levels of the dryer that were passed through. The invention of the patent application comes into play at this point.